Trimetal catalysts containing platinum (Pt), palladium (Pd), and rhodium (Rh) as main catalytic metals have been used for purification of exhaust gas discharged from a gasoline engine. Proposed examples of the trimetal catalysts include a catalyst including these three catalytic metals mixed in a single catalytic layer, and a double-layer catalyst including Pd contained in a lower catalytic layer, Rh contained in an upper catalytic layer, and Pt contained in at least one of the lower and upper catalytic layers. In addition, various other types of catalysts have also been proposed. Examples of other catalysts include a catalyst on which those catalytic metals are separately loaded on an upstream side and a downstream side in the flowing direction of an exhaust gas, a catalyst on which different catalytic metal species are loaded at a center portion and a peripheral portion of a honeycomb substrate, and a catalyst on which a catalytic metal specie is loaded in different concentrations at the center and peripheral portions of the honeycomb substrate.
As a next-generation engine combustion technology, attention has recently been paid to homogeneous charge compression ignition (HCCI) combustion. In the HCCI combustion, gasoline in a combustion chamber is compressed to the point of auto-ignition and combusted in a lean atmosphere in accordance with an operation state of the engine. An operation range of the HCCI combustion is limited at present due to constraints on maximum cylinder pressure (Pmax) and a rate of increase in cylinder pressure (dP/dθ). Thus, an engine has been developed in which a combustion mode is switched with a low-load range of the engine regarded as an operation range for the HCCI combustion, and with a high-load range of the engine regarded as an operation range for spark ignition (SI) in which a fuel is ignited by an ignition plug as an assistant igniter. The result of the inventors' study on the composition of the exhaust gas generated by the HCCI combustion revealed that the exhaust gas contained a relatively large amount of saturated hydrocarbons having a carbon number of 5 (n-pentane, i-pentane) and CO. A possible cause of the fact is that the fuel, which is gasoline, is combusted at low temperature.
Such saturated hydrocarbons are also contained in the exhaust gas discharged from a general engine in which the fuel is combusted around stoichiometry, although the amount of the saturated hydrocarbons is not as large as the amount contained in the exhaust gas discharged from the engine performing the HCCI combustion. Thus, when the temperature of the exhaust gas discharged from the general gasoline engine is as low as the temperature of the exhaust gas discharged when the engine is started, the catalytic metals are not activated yet, and thus the saturated hydrocarbons are just discharged before being oxidized and purified sufficiently.
Patent Document 1 presents, as a catalyst for oxidizing the saturated hydrocarbons, a hydrocarbon combustion catalyst obtained by loading a platinum group metal on silica-alumina in which an atomic ratio of aluminum (Al)/silicon (Si) is 5-60. According to Patent Document 1, if Pd is loaded as a platinum group metal on the catalyst, the catalyst promotes combustion of propane (C3H8) which is one of those saturated hydrocarbons. Such a catalyst is suitably used in a high temperature combustor using catalytic combustion, such as boilers, jet engines for aircrafts, gas turbines for automobiles, and gas turbines for power generation.